Gold(I) complexes bearing N-heterocyclic carbenes (NHC) of the type (NHC)AuBr (3a/3b) [NHC = 1-methyl-3-benzylimidazol-2-ylidene (= MeBnIm), and 1,3-dibenzylimidazol-2-ylidene (= Bn 2 Im)] are prepared by transmetallation reactions of (tht)AuBr (tht = tetrahydrothiophene) and (NHC)AgBr (2a/2b). The homoleptic, ionic complexes [(NHC) 2 Au]Br (6a/6b) are synthesized by the reaction with free carbene. Successive oxidation of 3a/3b and 6a/6b with bromine gave the respective (NHC)AuBr 3 (4a/4b) and [(NHC) 2 AuBr 2 ]Br (7a/7b) in good overall yields as yellow powders. All complexes were characterized by NMR spectroscopy, mass spectrometry, elemental analysis and single crystal X-ray diffraction. Reactions of the Au(III) complexes towards anionic ligands like carboxylates, phenolates and thiophenolates were investigated and result in a complete or partial reduction to a Au(I) complex. Irradiation of the Au(III) complexes with UV light yield the Au(I) congeners in a clean photo-reaction.

Unusual coordination for gold: an imidazolium salt was synthesized and used as a precursor for an N-heterocyclic carbene, which can be considered as the carbene analogue of 1,10-phenanthroline. Like the diimine congener, this ligand gives luminescent metal complexes. Remarkably, the Au(III) complex features a gold atom in an unusual environment: it is surrounded by six donor atoms, two of which interact electrostatically with the Au atom.

1-[2-(Dialkylamino)ethyl]-3-methylimidazolium salts (alkyl = Me (1a), i-Pr (1b)) have been prepared and used as precursors for the synthesis of the corresponding [(NHC)2Ag][AgCl2] complexes (NHC = N-heterocyclic carbene, Me (2a), i-Pr (2b)). Upon treatment of 2a with HBF4, crystals of the unprecedented, NHC-stabilized silver cluster [(NHC)4Ag10Cl10] (5) were obtained and characterized by X-ray diffraction. The crystal structure reveals that the carbene carbon atom exists in the rare μ2-coordination pattern, bridging two Ag(I) atoms with further stabilization of the cluster by numerous argentophilic interactions and a coordination of the amino nitrogen donor to one of the silver atoms. Transmetalation of 2a,b with (R2S)AuCl leads to the respective Au(I) complexes 3a,b, which are further oxidized with Br2 to (NHC)AuBr2Cl (4a,b). In red crystals of 4a the gold atom is coordinated in the unusual square-pyramidal geometry with the amine nitrogen atom in the axial position. Upon dissolution in wet organic solvents the amino group is protonated and the color changes to yellow. In square-planar Au(III) halide complexes electronic absorption spectra are dominated by LMCT absorption bands, but in the case of a square-pyramidal coordination sphere the d z 2 orbital is destabilized, becoming the HOMO and causing a low-energy d–d absorption. This interpretation is supported by DFT calculations.

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The reaction of meta- and para-bromomethylene-azobenzenes with 1-methyl-imidazole yields the respective meta-/para-functionalized azobenzenes tagged with an imidazolium group. Similar reactions of ortho- and para-bromo-azobenzene with imidazole and successive quaternation with benzylbromide give the analogues, with an imidazolium group in ortho/para substituted azobenzenes. With the exception of the ortho derivative, all imidazolium salts could be transformed into their respective silver(i) complexes by reaction with Ag2O. Transmetallation of these silver(i) complexes with (Me2S)AuCl gives the azobenzene-containing complexes (NHC)AuCl. Two of these formed crystals suitable for X-ray diffraction, which revealed the typical linear coordination geometry of the NHC-Au-Cl moiety. All gold complexes feature E→Z photo-isomerisation upon irradiation with UV light. The thermal back reaction to the E-isomers is relatively slow and comparable to that of other azobenzene compounds.

Six examples from a series of complex salts containing bis(1,3-dialkylimidazol-2-ylidene)silver(I) cations (with dialkyl = dimethyl, diethyl, methyl, ethyl, diisopropyl) with [PF 6 ] − , [SbF 6 ] − , [ClO 4 ] − , or [AgBr 2 ] − anions, respectively, were prepared in high yields and characterized by elemental analysis, 1 H and 13 C NMR spectroscopy, and mass spectrometry. Single-crystal X-ray diffraction experiments reveal unprecedentedly large contractions of the metallophilic Ag(I)−Ag(I) distances in the solid-state structures upon cooling. In the salt containing diisopropylsubstituted ligands and [PF 6 ] − anions, a contraction from 3.498(8) to 3.180(2) Å was observed on cooling from 293(1) to 100(1) K. Photochemical measurements show strong hypsochromic shifts of the emission maxima upon cooling, underlining the metallophilic-based nature of the emission bands. Ab initio calculations show that the strong temperature dependence of the observed Ag−Ag distances can be attributed to some extent to both the shallowness and the anharmonicity of the intermetallic interaction potential. The Ag−Ag interaction potentials are found to be attractive only when relativity is accounted for in the calculations.

In the trinuclear, heterometallic cluster compound [AuCuCl(P∩N)]PF metallophilic interactions give rise to very efficient cold-white light emission as a result of at least two thermally non-equilibrated emissive triplet states (one of mainly Cu → py and the other of Au → py character, respectively) with exceptional spin-orbit coupling and short emission lifetimes, which are competitive to Pt- and Ir-based emitters.